Interactive Map of Iowa County Formation History
This is a really interesting map that can show you how the counties in Iowa were formed over time. Simply click on the "play" button and the maps will transform over the years with each year posted and details show. Fascinating.
Atlas of Marion County, Iowa, 1901
From the University of Iowas libraries, there are five map objects: introductory pages, township maps, business directories, county history, and finally maps of Iowa, United States, territories, and the world
Atlas and Plat Book of Marion County, Iowa, 1917
Similar to the 1901 above but for 1917. One interesting page is a list of Iowa towns and places with population values, key to find it on the Iowa map and where telegraph offices and railroad offices are and also county seats. The Iowa map also shows all the railroad lines.
Plat Book of Marion County, Iowa, 1930
As the previous two, this is also a collection of maps housed at the library at the University of Iowa. It is a bit simpler than the previous two and also set in 1930: cover, map of the county as a whole and then township plat maps.
Atlas and Rural Directory of Marion County, Iowa, 1937
Another collection held at the the Iowa Digit Library at the University of Iowa, but for 1937. Contents: covers; overview map of county and list of business and professional listings; then listing of names per township listing spouse and if renting (and then gives owner) and other details; next are plat maps for each township; finally maps of the towns of Pella and Knoxville.
Another source of 1901 Plat maps
From the site, plat maps for each township are given and some townships over two maps.
Marion county GIS maps
These maps are in pdf format - many current street/highway, city, cemetery, park, election, historic, and other maps (and links to adjacent counties). I counted at least 62 maps. Great for any map geek!